Tonometry:a study in biomechanical modelling. Appraisal and utility of measurable biomechanical markers.

Abstract

Goldmann Applanation Tonometry (GAT) is the recognised ‘Gold Standard’ tonometer.However this status is refuted by eminent authors. These contradictory views have driventhe initial goal to assess, from first principles, the evolution of GAT and to experimentallyevaluate its utility and corrections. Subsequently, an important caveat became theevaluation of Corneal Hysteresis and Corneal Resistance Factor.Chapter 1. Biomechanical building blocks are defined and constitutive principlesincorporated into continuum modelling. The Imbert-Fick construct is re-interpreted asimple biomechanical model. GAT corrections are also appraised within a continuumframework; CCT, geometry and stiffness. These principles enable evaluation ofalternative tonometer theory and the evolving biomechanical markers, CornealHysteresis (ORA-CH) and Corneal Resistance Factor (ORA-CRF).Chapter 2 appraises corneal biomechanical markers, CCT, curvature, ORA-CH andORA-CRF in 91 normal eyes and the impact these have on three tonometers: GAT,Tonopen and Ocular Response Analyser (ORA). Tonopen was the sole tonometer notaffected by biomechanics. CCT was confirmed the sole measurable parameter affectingGAT. ORA did not demonstrate improved utility. ORA-CH and ORA-CRF do not appearrobust biomechanical measures.Chapter 3 assessed agreement between GAT, the ORA measures and Tonopen.Tonopen is found to measure highest and raises the question should a development goalemphasise GAT agreement or improvement?Chapter 4 assessed repeatability of the three tonometers and biomechanical measureskeratometry, pachymetry, ORA-CH and ORA-CRF on 35 eyes. Coefficients ofRepeatability (CoR) of all tonometers are wide. Effects assessed in Chapter 5 may bemasked by general noise. ORA does not appear to enhance utility over GAT.Isolation of corneal shape change via Orthokeratology (Chapter 5) demonstrate ORACHand ORA-CRF reflect, predominantly, a response to corneal flattening. It is proposedthey do not significantly reflect corneal biomechanics.After reviewing models for tear forces (Chapter 6), a refined mathematical model ispresented. Tear bridge attraction is minimal and cannot explain under-estimation of IOPby GAT in thin corneas. CCT corrections and the Imbert-Fick rules are incompatible.Chapter 7 summarises findings. The supremacy of GAT is likely to remain for some time,reflecting the sheer magnitude of overturning 60 years of convention, historicalprecedent, expert opinion as well as the logistical and educational difficulties ofredefining standards and statistical norms.